When steam is heated to a temperature above the boiling point corresponding to its pressure, it becomes superheated steam. Superheated steam has a greater heat capacity than air due to its water content and is therefore capable of delivering more heat energy to an instrument to be dried, thus vaporizing moisture more efficiently. This property of superheated steam has been exploited in a number of industries including the food and paper industries.
In known steam sterilization processes which are used to sterilize medical or dental instruments and the like, unless they are to be used immediately, the instruments must be dried after they have been sterilized. Air (hot or room temperature) is often used to perform this drying step. Heat energy from the air is transferred to the instruments to vaporize any residual moisture on the instruments or within the sterilization chamber. Residual moisture on the instruments is also evaporated by the release of the heat stored in the instruments, in which case the air acts as a carrier to remove the evaporated moisture. The use of air for drying is not particularly effective if the instruments are wrapped or pouched or if the instruments have a shape that traps moisture. In particular, sterilization pouches which are typically used in the medical and dental industries tend to retain water and air drying of these pouches can be both ineffective and time-consuming.
There are also known steam sterilization processes which involve vacuum-assisted drying where chamber pressure is lowered by means of drawing a vacuum to decrease the boiling point of the condensate causing it to evaporate more rapidly. This process requires a vacuum pump in addition to heating means which is costly and requires a leak-tight system to operate.
U.S. Pat. No. 6,026,588 discloses a superheated vapour dryer system for the precision removal of water from parts including disk drive media, flat panel displays and the like. The system boils and condenses solvents such as isopropyl alcohol to remove water and other contamination from the parts. After the parts have been treated with a liquid solvent, they are exposed to superheated vapours. As heat from the superheated vapours is transferred to the parts, any liquid solvent remaining will be boiled off. The parts are supported by a moving tray or suspended from hooks so that they may be immersed in and removed from a supply of liquid solvent. The parts are exposed to the superheated vapours by moving them through a so-called ‘vapour zone’. This type of system is not well-suited to the medical/dental sterilization industry in terms of size and speed requirements.
A number of prior patents in the field of medical/dental sterilization disclose the use of superheated steam for sterilization. U.S. Pat. No. 1,902,625 (Dunham) discloses a steam sterilizer in which superheated steam is fed into a sterilization chamber via a plurality of expansion nozzles to sterilize instruments. U.S. Pat. No. 1,377,725 (Pentz) also discloses a steam sterilizer in which steam at a temperature ‘high above the boiling point’ is fed into a sterilization chamber via a plurality of openings in the walls of the sterilization chamber. As the superheated steam cools after sterilization is complete, it will condense on Instrument surfaces.